A Raster Output Scanner (ROS), as is well known in the document reproduction arts, outputs a digital image onto a photoreceptive surface. Operationally, the ROS repeatedly scans a modulated beam over the photoreceptor surface in accordance with a predetermined pattern of an image. A conventional ROS typically includes a laser diode to generate the light beam that is modulated in response to received data and a rotating polygonal mirror to reflect the modulated beam across the photoreceptor surface as the photoreceptor surface is advanced. Each scan typically traces a single scanline across the photoreceptor surface in a direction that is substantially normal to the movement of the photoreceptor. Variations, for instance, in the angular speed of the rotating polygonal mirror, as well as variations in the geometry of the sidewalls or facets of the polygonal mirror, can distort the trajectory of the beam scan. Such distortions are known in this art as bow and skew.
Attention is directed to prior art FIG. 11A which shows a well-known basic configuration of a scanning system used, for example, in an electrophotographic printer or facsimile machine. Laser source 1110 produces a collimated laser beam 1112 which is reflected from the facets of a rotating polygon 1114. The polygon surface deflects the laser beam to direct a line of beam scanning 1116 toward the photoreceptor 1118. When the line of beam scanning 1116 reaches the photoreceptor, a scanning line 1120 results across the photoreceptor. The periodic scanning of beam 1116 across the rotating receptor creates a raster, or array of scan lines, on the photoreceptor, creating the desired image to be printed. Such a configuration typically includes any number of lenses and mirrors to accommodate a specific design. Unavoidable imprecisions in the shape and/or mounting of these optical elements will inevitably introduce certain anomalies in the quality of the scan line on the photoreceptor, and consequently create flaws in the printed document. Two important types of such anomalies are “skew” and “bow.” Skew is the error in rotational orientation of scan lines relative to the photoreceptor. As shown in FIG. 11, the scan line 1120 is rotated slightly relative to a line 1122, which is parallel with the axis of the photoreceptor 1118. If the photoreceptor is a plate or belt, the scan line may be skewed relative to an important base line, such as a line perpendicular to the edge of the belt. Further, if a number of rasters are superimposed on a document, as in a color copier, the different skews of the different rasters will cause a noticeable interference pattern on the document, to the great detriment of copy quality. Bow is the quality of a scan line to form not a straight line on the photoreceptor, but a line which bows about a central midpoint. An example of bow is shown by scan line 1120′ in FIG. 11B. Even in a monochromatic printer, a pronounced bow of the lines in a raster will be noticeable. In a color printer or copier, the different extent and/or direction of bow for each superimposed color raster can be an important cause of a conspicuous color banding on the document. Depending on the types of imprecision in the construction of the apparatus, the bow may bend in either direction relative to the center line 1122. In manufacturing situations, it is also very common to have both skew and bow simultaneously evident in the scan line 1120.
To eliminate such distortions the beam scan trajectory must be within relatively tight tolerances, for example, ±5μ. Tolerances on a micron level are difficult to achieve solely by opto-mechanical means. As such, electronic registration systems to compensate for skew and bow errors have been introduced in this art. Electronic registration systems manipulate the image data such that the resulting output image contains no such beam scan trajectory distortions, or such distortions have otherwise been minimized to a visually acceptable level. Electronic registration (ER) systems save cost over opto-mechanical means.
However, ER systems in document reproduction devices capable of high speed, high volume reproduction require expensive high speed buffers sufficiently large enough to store enough of the image data to span the range of the bow and skew error such that the distortions in the beam scan trajectory can be corrected or otherwise compensated for. For example, in system capable of 2400 dots-per-inch resolution (1 bit/pixel) and having a beam scan trajectory distortion of 2 millimeter across the trajectory path, a total of 189 full width scanlines of image data need to be buffered for correction. This approximates to 5.6 Mbits or ≈100 high speed buffers per millimeter of compensation. Such a large buffer is expensive and moreover greatly increases onboard or on-chip requirements to handle such a buffer size. If the buffering required by an ER system sufficient to compensate for a sufficiently large amount of skew and bow is large, the hardware requirements, in terms of FPGA, ASIC, and the like, as are common in ER systems, can become cost prohibitive. Since high speed buffers quickly become prohibitive from a cost and space-constraint perspective, systems and methods are needed in this art which reduce scanline buffer size requirements in many electronic registration applications.
Accordingly, what is needed in this art are increasingly sophisticated systems and methods which reduce on-board or on-chip memory buffer size requirements in electronic registration systems correcting for beam scan trajectory distortions in a digital image.